The present invention relates to magnetic disk cartridges, and methods for manufacturing magnetic disk cartridges. More particularly, the invention relates to a magnetic disk cartridge which is free from the difficulty that the color of the cartridge is made nonuniform by weld lines which are liable to be formed in the injection molding of a colored synthetic resin, thus impairing the external appearance of the cartridge, and to a method of manufacturing the magnetic disk cartridge.
Disk cartridges employed for micro floppy disks or optical disks are composed of upper and lower half shells relatively small in thickness which are formed by injection molding acrylonitrile-butadiene-styrene resin (hereinafter referred to as "ABS resin" when applicable) or the like.
The upper and lower half shells are formed by first defining a metal mold cavity corresponding to the configuration of a half shell to be formed by upper and lower halves of a metal mold, and then injecting a molten material into the metal mold cavity thus defined through injection gates such as direct gates and submarine gates. The upper and lower half shells are, in general, improved in external appearance by coloring them. For this purpose, a coloring method such as a colored pellet method or a master batch method is employed. More specifically, the resin material to be injected into the metal mold is colored in accordance with the coloring method in advance, and the resin material thus colored is supplied to the injection molding machine to form colored half shells.
In the case of the colored pellet method, natural pellets (stock resin), pigment, and dispersant are mixed together in advance, and the resultant mixture is kneaded with a single or twin screw extruder to form colored pellets. The colored pellets thus formed are used for the injection molding operation. Thus, the method is advantageous in that the pigment is dispersed with high uniformity; however, it is still disadvantageous in that the coloring cost is high.
The aforementioned master batch method employs a pellet-shaped coloring agent called "master batch" in which a pigment having a desired color is mixed to a high density. In the case a molding operation, the master batch is mixed with natural pellets, and the resultant mixture is diluted so as to be colored. The master batch method is advantageous in that, when compared with the above-described colored pellet method, the coloring cost is low, and the automatic weight and mixing operation can be achieved with ease.
In general, in forming upper and lower half shells which are relatively small in thickness by injection molding, the molten resin is injected into the metal mold through two or four gates. In this case, weld lines are formed in the metal mold where at least two streams of molten resin meet. In other words, when the streams of molten resin meet, because the forward portions of the streams of molten resin have already decreased in temperature, they are not sufficiently joined together, thus forming weld lines. The portion including such weld lines is lowered in mechanical strength, often being cracked or deformed. In addition, the portion is liable to contain air pockets or to contain gas, thus further impairing the external appearance of the product.
Particularly in the case where the resin is colored by the master batch method, the kneading operation is carried out only with the screw of the molding machine, so that the degree of kneading is low. Therefore, in this case, the dispersion and adhesion of the pigment with respect to the natural pellets are lower than in the case where the colored pellet method is employed. Hence, when the pigment is localized, being secondarily condensed by the gas pressure, the portion where weld lines are formed is liable to be low in color uniforimity, thus impairing the external appearance. Particularly in the case where ABS resin is employed for the molten resin, the dispersion of pigment is insufficient because of the low adsorption of the butadiene and pigment in the resin. Thus, the product often suffers from color nonuniformity.